The plant treats in excess of 3,600 million gallons/year; with a population
equivalent of 250,000 with industry loadings. The Nampa Wastewater Treatment
Plant removes 99% of BOD (Biological Oxygen Demand), 96% of TSS (Total Suspended
Solids) and 99.6% of NH3-N (Ammonia).
Wastewater flows by gravity to the
Plant Headworks where it is lifted by the Raw Sewage Pumps. Then Step screens
remove sticks and rags from the raw sewage flow. Grit basins remove inorganic
grit and snail shells. Rag and grit removal reduces possible plugging problems
and prevents excess, while also preventing excessive pump wear.
From the grit basins, the flow is split through three flow meters which
measure the raw sewage flow. The flow from each parshall flume goes to a primary
Primary clarification removes settleable (sludge) and floating (scum)
Effluent from the primary clarifiers flows to the primary
effluent splitter box where the flow is split either to the trickling filters or
to bypass the trickling filters. The amount of flow that goes to the trickling
filters is limited by their treatment capacity.
The flow that goes to the
trickling filters is diluted with returned trickling filter effluent to minimize
shock loading. The mixture is then pumped to an elevated wet well where the
mixture is split to the three trickling filters. The trickling filters provide
biological treatment by means of a fixed microbial mass living on the rocks
(media). The flow is evenly spread over the media by the trickling filter
mechanism. The flow that passes through the trickling filters returns to the
influent, while the rest goes to the trickling filter effluent pump station.
This pump station uses screw pumps and lift pumps to lift the flow and then
splits the flow to the two secondary clarifiers. These clarifiers remove the
solids that grew in the trickling filters and have fallen (sloughed) off of the
The effluent from the secondary clarifiers goes to the secondary
effluent pump station where it combines with the flow that bypassed the
trickling filters. This pump station also uses screw pumps to lift the flow and
then splits the flow to the Nitrification Basins. Inside the Nitrification
Basins the flow is mixed with living organisms contained in the return activated
sludge. The nitrification basins provide adequate detention time for the
organisms to perform biological treatment which removes oxygen demanding
pollutants and also converts ammonia nitrogen to the nitrate form. The
Nitrification Basin has two halves with six mixed areas in each half. Air is
added to each mixed area to give the organisms air to breathe.
Nitrification Basin effluent continues to the final clarifier splitter box. Here
the flow can be split between the three final clarifiers as needed. The final
clarifiers capture the organisms grown in the Nitrification Basin and also any
scum that gathers during treatment. Downstream of the final clarifiers, the flow
goes through the chlorine mixing station where sodium hypochlorite is thoroughly
mixed with the flow. The flow then goes through the chlorine contact chamber
where bacteria are killed by the hypochlorite. The hypochlorite is then
neutralized by sodium bisulfite that is added at the #4 Water Pump
The flow then goes to the post aeration basins where air is
added to raise the dissolved oxygen level in the wastewater and a defoaming
agent is added to control foaming on the creek. The flow then goes to Indian
Creek via the plant outfall.
rags are removed by the Step Screens and stored in the Grit Processing Building
before being removed by the local sanitation service.
Grit removal at the
Grit Basins are pumped to the Grit Processing Building where it is separated and
stored before being removed by the local sanitation service.
Primary Sludge is pumped to the to the Primary Digesters. The
Primary clarifier Effluent flows to the bypass splitter box where flow can send
either to the trickling filters or directly to the Nitrification Basins. Primary
scum is pumped directly to the primary digesters.
Clarifier sludge is returned to the plant flow upstream of the Grit
Basins. It is normally sent to the Grit Basins in order to remove the snail
shells that grow in the Trickling Filters. The organic matter then settles out
in the Primary Clarifiers.
The Final Clarifier sludge is
returned to the Nitrification Basins as return activated sludge. This sludge
provides the living organisms needed to treat the sewage flow in the
The wasting of activated sludge comes from the
return activated sludge pumps. This waste activated sludge is pumped to the
flotation thickener where it is thickened. It can also be directly sent back to
the headworks where it settles in the primary clarifiers.
sludge is then pumped to the primary digesters. The thickener effluent flows to
the secondary effluent pump station. The scum that is collected from the final
clarifiers also goes to the primary digesters.
In the primary digesters,
sludge and scum are treated. This process involves heating and mixing the sludge
in the absence of air (anaerobic). The volume of sludge is reduced, and
pathogens are reduced or eliminated in the process. Methane gas produced during
anaerobic digestion is burned in the four hot water boilers and also in the
engine generator. Hot water from the boilers is used to heat and maintain the
digester sludge temperature and to heat the Administration Maintenance Building.
The digester gas is also used to mix the sludge in the primary digesters. Excess
gas, if any, is burned in a waste gas burner.
Primary digested sludge
flows by gravity to the secondary digesters where the sludge can settle.
Secondary digested sludge flows either to the drying beds, the sludge holding
tank, or to the blower building for belt press dewatering. Sludge in the sludge
holding tank can be pumped to drying beds.